Seismic sources are used to introduce controlled acoustic waves into the subsurface such that the direct and/or reflected waves can be defected by sensitive sensors at the surface or downhole. The detected energy is typically collected and processed and used to develop images of the subsurface. These subsurface images are regularly used by oilfield companies to locate, monitor, and enhance the recovery from subsurface oil & gas deposits.
In general, there are two types of seismic sources used for exploration and production in a land oilfield, those placed on the surface and those placed in the subsurface, i.e., downhole. The most widely used oilfield surface seismic source is the “vibroseis truck” which lowers a heavy plate onto the earth's surface which vibrates to radiate sound waves info the ground. The disadvantages of surface seismic sources such as the vibroseis truck include:                limited frequency bandwidth        relatively large size        difficult to use in non-level, rocky, or muddy terrain        high purchase price or high daily rental expense        surface signal is attenuated and filtered by the relatively “lossy” near-surface layers        can cause structural/vibrational damage to surroundings when used in populated areas        
Downhole sources were originally developed to get away from some of the problems associated with surface sources, but downhole sources introduced problems of their own, namely:                limited power output (to prevent causing damage to well casings/cement bonds) thereby limiting transmission distance        difficult to achieve good acoustic coupling once lowered into position downhole        reliability issues associated with operating in rugged downhole environments that can include high temperatures and pressures        high cost of downhole equipment        not designed for permanent installation (most require periodic maintenance).        
By an overwhelming percentage, the majority of onshore seismic surveys performed today employ surface seismic, that is, both the seismic sources and the seismic receivers are located at the surface. For improved image resolution, leading oil companies now agree that both the sources and receivers need to be closer to the reservoir, which means downhole.
Geophysicist's acknowledge that they experience as much as a 99 percent attenuation of the energy produced by surface sources (i.e., vibroseis trucks) as it passes through the unconsolidated near surface (top 100-200 feet) layers, “Bypassing the attenuation of the near-surface layer can reduce the power needed by two to three orders of magnitude, i.e., 30-300 kW for a surface source to ˜300 W for a downhole source,” (ref: P. C. Leary and L. A. Walter—Geophysical Journal International, August 2005). In addition, the near surface layers absorb much of the higher frequency signals, limiting their useful frequencies to approximately 50 Hz and below. At 200 Hz, GPUSA's non-impulsive or distributed energy downhole seismic source can quadruple the spatial/temporal resolution, based upon a 4× increase (50 Hz to 200 Hz) in received frequency.
Current downhole seismic sources are typically deployed via wireline into a wellbore as a single element, and are designed to be moved to multiple locations within the wellbore, as needed for a particular seismic survey. At each location, the source is typically clamped inside the wellbore prior to activation of the seismic source to achieve good coupling with the earth for best performance. Alternatively, the source can hang freely within the wellbore (i.e., fluid coupled) but transmission beyond the wellbore will be somewhat degraded. Each time a survey is performed, a wireline truck and crew is required at a significant expense. The major advantage of using downhole sources and sensors (as compared to surface seismic) is much higher resolution of the subsurface lithology.